Currently there are at least four commercial ways to manufacture acetic acid. The fermentation of fruit (apples) or wood waste are the oldest. The reaction of acetylene with water to form acetaldehyde followed by air oxidation is still in use. Fermentation of ethanol to acetic acid is used when cheap ethanol is available. The reaction of methanol with carbon monoxide in aqueous solution is favorable when cheap methanol is available. The air oxidation of butane to give a multitude of products, approximately forty, including acetic acid is another method. All of these processes present the problem of reducing the water content of the acetic acid. Acetic acid boils at 118° C. and water at 100° C., but although they do not form an azeotrope, they are far from being an ideal mixture to separate. The separation of water from acetic acid by distillation becomes especially difficult at high concentrations of acetic acid and low concentrations of water.
To address the difficulty in reducing the water content of acetic acid at high acetic acid concentrations, several methods have been developed. One such method is azeotropic distillation as described in U.S. Pat. Nos. 5,980,696, 5,160,412, 2,412,215, 2,317,758, 2,275,802, 2,031,637, G.B. Patent Nos. 623,991, 587,269, and CA Patent Nos. 330,190 and FR 703003. Other methods include extractive distillation, as described in U.S. Pat. No. 5,167,774, and the passing of the mixture through a column of molecular sieves. Some recent methods describe the dehydration of the acetic acid using semi-permeable membranes and pervaporation. However, these methods all have negatives associated with them such as requiring high amounts of energy, not adequately reducing the water content for certain applications, cost, and potentially requiring large capital investment to practice on a commercial scale.
The inventors have found that wet acetic acid can be dehydrated by contacting wet acetic acid with acetyl chloride. The inventors have also found that wet acetic acid may be dehydrated by contacting wet acetic acid with acetic anhydride and with acetyl chloride, hydrogen chloride, or a chlorosilane. The new methods offer the benefits of being able to be conducted in a standard batch reactor or column, reduction of wet acetic acid water content to parts per million levels, and relatively low energy requirements.